This invention regards a tensioning device for production tubing installed in a riser connecting a subsea hydrocarbon well with a floating installation on the surface of the sea, in particular a system for tensioning the fixed part of the production tubing and relieving it by suspending it from the fixed part of the riser, and also maintaining tension in that part of the production tubing which passes through a telescopic riser unit up to the production deck, by connecting a counterweight to the production tubing by means of a wire suspended from a pulley in the derrick.
During test production from a subsea hydrocarbon well by use of a floating installation in the form of a drilling platform or a drilling ship, the well and the floating installation are usually interconnected by several connecting pipes, among other things production tubing through which hydrocarbons flow up from the well. Several pipes may be gathered in a so-called riser of a dimension suitable for the purpose.
The actual riser is kept under tension by means of a riser tensioner attached to the floating installation. In order to compensate for movements of the floating installation caused by waves, tide, ballast trimming etc., the upper section of the riser comprises a telescopic section, which in some cases is combined with the riser tensioner. Some of the pipe connections inside the riser may have a similar telescopic section.
Under normal circumstances, it is not desirable for the production tubing to comprise a telescopic section. As a result, this is normally passed rigidly through the telescopic section of the riser and up to the production deck of the installation, where it is suspended from the tensioning system via wires, which tensioning system consists of pulleys and cylinders interconnected in a manner such as to maintain the tension in the production tubing regardless of the movements of the installation on the surface of the sea.
If the production tubing is not kept under tension, it may buckle under its own weight and possibly collapse, causing leakage. For this reason, the demands placed on the tensioners used on production tubing during this test phase are quite stringent. At the same time, the tensioners often comprise large, complex components that are demanding in terms of maintenance and which require a lot of space in the central areas around the production deck on the installation. For reasons of safety, it is often made use of dual systems. Consequently, the production tubing tensioners also represent a great load on the installation, with a relatively high centre of gravity.
The object of the invention is to remedy the disadvantages of prior art.